Currently, the IC manufacturing process involves more than two photomasks of different patterns to perform a double patterning. Therefore, the alignment between the photomasks determines the quality of patterns transferred to the target layer and the final performance of the IC.
FIG. 1A shows a top view of a pattern of a conventional photomask. The pattern 1 has a first rectangular region 11, a second rectangular region 12, a third rectangular region 13 and a fourth rectangular region 14. The longer side of the first rectangular region 11 and the longer side of the third rectangular region 13 are parallel to each other, while the longer side of the second rectangular region 12 and the longer side of the fourth rectangular region 14 are parallel to each other. The longer side of the first rectangular region 11 or the third rectangular region 13 is perpendicular to the longer side of the second rectangular region 12 or the fourth rectangular region 14. Therefore, along the horizontal and vertical directions, there are two parallel and symmetrical rectangular regions.
FIG. 1B shows a top view of an overlay mark in a substrate. The overlay mark 2 is formed on the substrate after the previous process is completed. The overlay mark 2 includes a first aligned rectangular region 21, a second aligned rectangular region 22, a third aligned rectangular region 23 and a fourth aligned rectangular region 24.
FIG. 1C shows a top view of an alignment configuration. The pattern 1 in FIG. 1A is transferred on a photoresist layer on the substrate to form a mark pattern 1a. The mark pattern 1a has a first rectangular region 11a, a second rectangular region 12a, a third rectangular region 13a and a fourth rectangular region 14a. A metrology process is performed to determine the alignment precision by referring to the overlay mark 2 and the mark pattern 1a on the photoresist layer. Specifically, by measuring the gap between the first aligned rectangular region 21, the second aligned rectangular region 22, the third aligned rectangular region 23 and the fourth aligned rectangular region 24 and the first rectangular region 11a, the second rectangular region 12a, the third rectangular region 13a and the fourth rectangular region 14a, the alignment step is performed. If the measured gap meets the predetermined criterion, the patternization is successful and process continues. However, if the criterion is not met, the failed photoresist layer at this stage must be removed and the lithography process is repeated again until the criterion is met.
FIGS. 2A to 2G show a conventional method for forming an overlay mark in a substrate. In another prior art, the overlay mark 2 is constituted by a plurality of hollow cylinders 36 (FIG. 2G), which are formed as described below. Referring to FIG. 2A, a photoresist layer 31 is applied on a substrate 30. Referring to FIG. 2B, a photomask 32 is provided. The photomask 32 comprises a plurality of patterns, and the patterns comprise a plurality of square areas 33. Each of the square areas 33 has the same light transmittancy, and the light transmittancy of the square areas 33 is different from that of the other area of the photomask 32. Usually, the square areas 33 are light transmissive, and the other area of the photomask 32 is opaque.
Referring to FIGS. 2C and 2D, wherein FIG. 2C is a top view of FIG. 2D, an exposure and development process is performed so that the photoresist layer 31 has a plurality of mark patterns. The mark pattern comprises a plurality of holes 34. Referring to FIG. 2E, a plurality of spacers 35 are formed on the sidewalls of the holes 34.
Referring to FIG. 2F, the photoresist layer 31 is removed, and the spacers 35 remain on the substrate 30. Referring to FIG. 2G, the substrate 30 is etched to form an overlay mark 2 corresponding to the spacers 35. The overlay mark 2 includes a plurality of hollow cylinders 36. The material of the spacers 35 is metal oxide, therefore, in the etching process, the spacers 35 can serve as a mask.
FIG. 2H shows a cross-sectional view of a conventional overlaying structure on the substrate of FIG. 2G. A priming step is performed to apply an adhesive layer 37 on the substrate 30. Next, a second photoresist layer 38 is applied on the adhesive layer 37 to adhere to the substrate 30. Then, an exposure and development process is performed, so that the second photoresist layer 38 has a plurality of second mark patterns 39 which are the same as the mark pattern 1a in FIG. 1C.
The second mark patterns 39 are located over the overlay mark 2. Therefore, a metrology process is performed to determine the alignment precision by referring to the second mark patterns 39 and the overlay mark 2. As shown in FIG. 2H, the measured distance d1 provided by the second mark patterns 39 and the overlay mark 2 can be used to perform the alignment procedure.
The drawback of the overlaying structure of FIG. 2H is as follows. The spacers 35 and the hollow cylinders 36 have the same thickness T1, which is very thin. Therefore, when the metrology process is performed, the contrast is low, and it is very difficult to find the overlay mark 2.